1. Field of the Invention
The present invention relates generally to optical waveguide devices, and particularly to optical waveguide devices that include negative thermal expansion substrates which athermalize an optical waveguide. The substrate is made from a material having a negative thermal expansion so that the substrate shrinks with a rise in temperature which compensates for thermally varying optical properties of the optical waveguide device.
2. Technical Background
This invention relates to a temperature compensated, athermal optical device and, in particular, to a stabilized negative expansion substrate for utilization in an optical fiber reflective Bragg grating optical waveguide device and to a method of producing the stabilized athermalizing optical waveguide substrate.
Index of refraction changes induced by UV light are useful in producing complex, narrow-band optical components such as filters and channel add/drop devices.
These devices can be an important part of multiple-wavelength telecommunication systems. A popular photosensitive optical fiber device is a reflective grating (Bragg grating), which reflects light over a narrow wavelength band. Typically, these devices have channel spacings measured in nanometers.
There are already known various constructions of optical filters, among them such which utilize the Bragg effect for wavelength selective filtering. U.S. Pat. No. 4,725,110 discloses one method for constructing a filter which involves imprinting at least one periodic grating in the core of the optical fiber by exposing the core through the cladding to the interference pattern of two ultraviolet beams that are directed against the optical fiber at two angles relative to the fiber axis that complement each other to 180xc2x0. This results in a reflective grating which is oriented normal to the fiber axis. The wavelength of the light reflected by such an optical fiber with the incorporated grating filter is related to the spacing of the grating which varies either with the strain to which the grating region is subjected, or with the temperature of the grating region, in a clearly defined relationship, which is substantially linear to either one of these parameters.
For a uniform grating with spacing L, in a fiber with an effective index of refraction n and expansion a, the variation of center reflective wavelength, lr is given by
dlr/dT=2L[dn/dT+na]
In silica and germania-silica fiber reflective gratings the variation in center wavelength is dominated by the first term in the brackets, the change of index of refraction with temperature. The expansion term contributes less than ten percent of the total variability. dlr/dT is typically 0.01 nm/xc2x0 C. for a grating with a peak reflectance at 1550 nm.
One practical difficulty in the use of these gratings is their variation with temperature. In as much as the frequency of the light reflected by the fiber grating varies with the temperature of the grating region this basic filter cannot be used in applications where the reflected light frequency is to be independent of temperature. Methods of reliably and stably athermalizing the fiber reflective grating are needed to meet the rigorous and always growing optical telecommunications application demands and requirements for such gratings.
One method of athermalizing a fiber reflective grating is to thermally control the environment of the grating with an actively controlled thermal stabilization system. Such thermal stabilization is costly to implement and power, and its complexity leads to reliability concerns.
A second athermalization approach is to create a negative expansion which compensates the dn/dT. Devices which employ materials with dissimilar positive thermal expansions to achieve the required negative expansion are known.
U.S. Pat. No. 5,042,898 discloses a temperature compensated, embedded grating, optical waveguide light filtering device having an optical fiber grating. Each end of the fiber is attached to a different one of two compensating members made of materials with such coefficients of thermal expansion relative to one another and to that of the fiber material as to apply to the fiber longitudinal strains, the magnitude of which varies with temperature in such a manner that the changes in the longitudinal strains substantially compensate for these attributable to the changes in the temperature of the grating.
Yoffe, G. W. et al in xe2x80x9cTemperature-Compensated Optical-Fiber Bragg Gratingsxe2x80x9d OFC""95 Technical Digest, paper WI4, discloses a device with a mechanical arrangement of metals with dissimilar thermal expansions which causes the distance between the mounting points of an optical fiber to decrease as the temperature rises and reduce the strain in a grating.
Another method of athermalizing optical waveguide devices utilizes a substrate for attachment with the optical fiber grating with the substrate fabricated from a material with an intrinsic negative coefficient of expansion.
One aspect of the present invention is a method of making a negative thermal expansion substrate which includes the steps of: providing a dimensionally unstable negative glass-ceramic with microcracks which have lengths and crack tips; driving the microcracks so as to increase the length of the microcracks of the provided glass-ceramic; and sealing the crack tips of the driven microcracks that have an increased length.
In another aspect, the present invention includes a method of making a negative thermal expansion substrate including the steps of: providing a negative expansion glass-ceramic having microcracks, and submerging the glass-ceramic in a liquid bath having a temperature less than 30xc2x0 C.
In a further aspect the invention comprises a method of making a negative thermal expansion substrate with the steps of: providing a negative expansion glass-ceramic having a plurality of microcracks, and exposing the glass-ceramic for at least twelve hours to a humid atmosphere having a relative humidity of at least 80% and a temperature of at least 80xc2x0 C.
In another aspect, the present invention includes a negative thermal expansion substrate for athermalizing an optical waveguide device for use in an optical waveguide deployment environment. The substrate includes a microcracked negative thermal expansion glass-ceramic body having a dimensional length, with the glass-ceramic body having a plurality of microcracks, with the microcracks having a stabilized saturated subcritical crack growth length wherein the dimensional length of the glass-ceramic body is stabilized when exposed to the optical waveguide deployment environment.
In a further aspect the invention includes a negative thermal expansion substrate for athermalizing an optical waveguide device, with the substrate including a microcracked negative thermal expansion glass-ceramic body having a plurality of microcracks terminating with a crack tip wherein the microcrack crack tips are substantially filled with a percipitant of glass-ceramic constituents leached from the glass-ceramic body.
The inventive substrates provide a high degree of dimensional stability under the long term exposures of humidity at various temperatures experienced with optical waveguide deployment environments.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying figures are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The figures illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.